Currently, the majority of autonomous and mobile electronic systems are powered by electrochemical batteries. Although the quality of these batteries has substantially improved over the last two decades, their energy density has not greatly increased. At present, limitations such as cost, weight, limited service time and waste disposal problems intrinsic to the materials used to form electrochemical batteries are impeding the advance of many areas of electronics. The problem is particularly acute in the area of mobile electronic devices, where rapidly-growing performance and sophistication of these devices leads to ever-increasing power demands—demands that cannot easily be met by traditional electrochemical batteries.
One of the technologies that holds great promise to substantially alleviate the current reliance on electrochemical batteries is high-power energy harvesting. The concept of energy harvesting works toward developing self-powered devices that do not require replaceable power supplies. In cases where device mobility is required, and high power consumption is anticipated, harvesters that convert mechanical energy into electrical energy are particularly promising as they can tap into a variety of high-power-density sources, including mechanical vibrations.
High power harvesting of mechanical energy is a long-recognized concept that has not been significantly commercialized to date, based on the lack of a viable energy harvesting technology. Existing methods of mechanical-to-electrical energy conversion such as, for example, electromagnetic, piezoelectric or electrostatic do not allow for effective direct coupling to the majority of high power environmental mechanical energy sources. In particular, bulky and expensive mechanical or hydraulic transducers are required by each of these existing methods to convert the broad range of aperiodic forces and displacements typically encountered in nature into a form useable for conversion to electricity.
An alternative approach to energy harvesting has recently been proposed that substantially alleviates the above-mentioned problems, the new approach being the use of a microfluidics-based energy harvester. In particular, an exemplary high power microfluidics-based energy harvester is disclosed in U.S. Pat. No. 7,898 issued to T. N. Krupenkin on Mar. 2, 2011, as well as U.S. Pat. No. 8,053,914 issued to T. N Krupenkin on Nov. 8, 2011, both of which are herein incorporated by reference. An exemplary embodiment of an energy harvester as described in the above-referenced patents generates electrical energy through the interaction of thousands of microscopic liquid droplets with a network of thin-film electrodes. A typical configuration of the Krupenkin energy harvester is capable of generating several watts of power.
An exemplary embodiment of this energy harvester is shown in FIG. 1, which illustrates a train of energy-producing conductive droplets 1 located along a microscopically-thin channel 2, where droplets 1 are suspended within a liquid dielectric medium 3 and are hydraulically actuated by applying a pressure differential between the ends of channel 2. Pluralities of separate electrodes 4-1 and 4-2 are disposed along either side of channel 2, which interact with droplets 1 as they move back and forth within channel 2 during changes in pressure. As conductive droplets 1 move along channel 2, they create arrays of capacitors with electrodes 4-1 and 4-2, the capacitors changing in stored charge as the droplets move back and forth, generating an electrical current flow along conductors 5-1 and 5-2. This type of hydraulic activation method provides an important advantage as it allows for efficient direct coupling with a wide range of high power environmental mechanical energy sources, including human locomotion.
While considered a significant advance in the field of energy harvesting, the arrangement as shown in FIG. 1 requires the use of an external source of bias voltage to generate the charges at electrodes 4-1 and 4-2. This bias voltage can be provided by sources such as electrochemical batteries or electrical capacitors. The output power density provided by the harvester device increases rapidly with larger bias voltages. Indeed, certain power density requirements may necessitate relatively high bias voltages (e.g., on the order of tens or even hundreds of voltages). The need to provide a bias voltage source may introduce unwanted complications in the design of the harvesting device and adversely affect its reliability.
Thus, a need remains in the art for an arrangement that provides the advantages of the microfluidic energy harvesting configuration as developed by Krupenkin without requiring the use of an external bias voltage source.